1. Field of the Invention
The present invention relates in general to a vehicle passive entry system, and more specifically, to power conservation in a remote keyless entry fob for a vehicle passive entry system.
2. Description of the Related Art
Remote entry vehicle systems are known in the auto industry for remotely accessing a vehicle without having to manually insert a key for unlocking the vehicle door. Such remote keyless systems are typically characterized as active or passive entry systems. Active entry systems require the user to manually activate a button on a portable communication device such as an RKE fob. The RKE fob when manually activated broadcasts a signal to a vehicle based communication module within the vehicle for receiving the signal and unlocking the vehicle accordingly.
Passive entry systems are passive in nature. That is, the user is not required to manually activate a button on the fob. In a polling passive entry system, the vehicle base communication module periodically transmits a challenge signal in the vicinity of the vehicle. When the fob is within a broadcast range of the challenge signal, the fob automatically broadcasts a response (RF) signal to the vehicle based communication module in response to the challenge signal. The vehicle based communication module receives the response signal, determines the authenticity of the fob, and actuates the vehicle unlock function accordingly.
Under normal operating conditions, the fob is powered at all times for sensing incoming signals from the vehicle base station. A receiver such as a low frequency (LF) receiver may be used for receiving a wakeup signal from the vehicle based communication module while all other components within the fob are in a sleep mode. This allows all other components within the fob to stay in a sleep mode thereby conserving power until a wakeup signal is received. When the portable communication device receives the wakeup signal, power is supplied to the RF transmitter for transmitting the challenge response signal. The portable communication device transmits a response (RF) signal in response to the challenge signal. Typically, very little power is consumed from the fob battery by the receiving circuits for continuous monitoring of the wakeup signal. A greater amount of power is consumed when the RF transmitter within the fob transmits a response signal in response to the challenge signal. Although the vehicle based communication module continuously transmits the wake up and challenge signals whenever the passive entry system is active, the fob is responsive to these signals only when the fob is within a broadcast reception region of the vehicle. As a result, battery drain under such conditions (i.e., when the fob is within the broadcast regions external to the vehicle) is minimal because such transmissions are infrequent and power is drawn only for a short duration of time (i.e., until the vehicle based communication module receives the broadcast signal and unlocks the vehicle doors). After activation of the passive entry system and the doors are unlocked, the vehicle based communication device terminates the broadcast of the wakeup signal and challenge signal.
If a fob is left within the interior region of the vehicle while the vehicle is locked and the engine is off (i.e., passive entry system active), then a significant drain on the fob battery may result as the fob will transmit a response signal each time a respective challenge signal is received. That is, if the vehicle based communication module identifies that the fob is within the interior of the vehicle, the vehicle door unlock function will not be actuated. The vehicle based communication module, however, will continue to transmit wake-up signals in the course of normal operation until a response signal is received from an authenticated fob located in the exterior to the vehicle. In the interim, the fob located within the interior of the vehicle will continue to transmit response signals and significant power drain on the fob battery may occur as a result.